Niobium fluoride: Lewis acidity

Perchloric acid (HCIO4 Ho —13.0), fluorosulfuric acid (HSO3F Ho — 15.1), and trifluoromethanesulfonic acid (CF3SO3H Ho —14.1) are considered to be superacids, as is truly anhydrous hydrogen fluoride. Complexing with Lewis acidic metal fluorides of higher valence, such as antimony, tantalum, or niobium pentafluoride, greatly enhances the acidity of all these acids. 

In a generalized sense, acids are electron pair acceptors. They include both protic (Bronsted) acids and Lewis acids such as AlCb and BF3 that have an electron-deficient central metal atom. Consequently, there is a priori no difference between Bronsted (protic) and Lewis acids. In extending the concept of superacidity to Lewis acid halides, those stronger than anhydrous aluminum chloride (the most commonly used Friedel-Crafts acid) are considered super Lewis acids. These superacidic Lewis acids include such higher-valence fluorides as antimony, arsenic, tantalum, niobium, and bismuth pentafluorides. Superacidity encompasses both very strong Bronsted and Lewis acids and their conjugate acid systems. 

On the other hand, the 25Nb AlF3 sample recovered after the separation of leached aluminum species became highly selective to hydroxy acids, with a preponderant formation of lactic acid, while the conversion of cellulose decreased only by a few percent when compared with the fresh catalyst (Table 6.9, entry 7). This catalytic performance improvement is clearly due to the leaching of aluminum that enriches the catalyst in niobium oxide fluoride phases (boiled catalyst contains 42.5 mol% Nb and 57.5 mol% A1 as against fresh catalyst (25 mol% Nb and 75 mol% Al)) alongside the introduction of an oxygen/aluminum vacancy adjacent to the niobium sites (NbOF - Lewis acid sites), by the formation of different sixfold coordinated Al units AlF Og. , fivefold coordinated units AIF 05, and fourfold coordinated species AlF 04 t- This way an optimum combination of acidity for the one-pot transformation of cellulose to lactic acid seems to be created. 

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